Historically, today's most favorite rechargeable energy storage devices—lithium-ion batteries—actually evolved from rechargeable “lithium metal batteries” using lithium (Li) metal or Li alloy as the anode and a Li intercalation compound as the cathode. Li metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). Based on these outstanding properties, lithium metal batteries were proposed 40 years ago as an ideal system for high energy-density applications. During the mid-1980s, several prototypes of rechargeable Li metal batteries were developed. A notable example was a battery composed of a Li metal anode and a molybdenum sulfide cathode, developed by MOLI Energy, Inc. (Canada). This and several other batteries from different manufacturers were abandoned due to a series of safety problems caused by sharply uneven Li growth (formation of Li dendrites) as the metal was re-plated during each subsequent recharge cycle. As the number of cycles increases, these dendritic or tree-like Li structures could eventually traverse the separator to reach the cathode, causing internal short-circuiting.
To overcome these safety issues, several alternative approaches were proposed in which either the electrolyte or the anode was modified. One approach involved replacing Li metal by graphite (another Li insertion material) as the anode. The operation of such a battery involves shuttling Li ions between two Li insertion compounds, hence the name “Li-ion battery.” Presumably because of the presence of Li in its ionic rather than metallic state, Li-ion batteries are inherently safer than Li-metal batteries.
Lithium ion battery is a prime candidate energy storage device for electric vehicle (EV), renewable energy storage, and smart grid applications. The past two decades have witnessed a continuous improvement in Li-ion batteries in terms of energy density, rate capability, and safety, and somehow the significantly higher energy density Li metal batteries have been largely overlooked. However, the use of graphite-based anodes in Li-ion batteries has several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g as opposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g. low solid-state diffusion coefficients of Li in and out of graphite and inorganic oxide particles) requiring long recharge times (e.g. 7 hours for electric vehicle batteries), inability to deliver high pulse power (power density <<1 kW/kg), and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathodes have a relatively low specific capacity (typically <200 mAh/g). These factors have contributed to the two major shortcomings of today's Li-ion batteries—low gravimetric and volumetric energy densities (typically 150-220 Wh/kg and 450-600 Wh/L) and low power densities (typically <0.5 kW/kg and <1.0 kW/L), all based on the total battery cell weight or volume.
The emerging EV and renewable energy industries demand the availability of rechargeable batteries with a significantly higher gravimetric energy density (e.g. demanding >>250 Wh/kg and, preferably, >>300 Wh/kg) and higher power density (shorter recharge times) than what the current Li ion battery technology can provide. Furthermore, the microelectronics industry is in need of a battery having a significantly larger volumetric energy density (>650 Wh/L, preferably >750 Wh/L) since consumers demand to have smaller-volume and more compact portable devices (e.g. smart phones and tablets) that store more energy. These requirements have triggered considerable research efforts on the development of electrode materials with a higher specific capacity, excellent rate capability, and good cycle stability for lithium ion batteries.
Several elements from Group III, IV, and V in the periodic table can form alloys with Li at certain desired voltages. Therefore, various anode materials based on such elements and some metal oxides have been proposed for lithium ion batteries. Among these, silicon has been recognized as one of the next-generation anode materials for high-energy lithium ion batteries since it has a nearly 10 times higher theoretical gravimetric capacity than graphite 3,590 mAh/g based on Li3.75Si vs. 372 mAh/g for LiC6) and ˜3 times larger volumetric capacities. However, the dramatic volume changes (up to 380%) of Si during lithium ion alloying and de-alloying (cell charge and discharge) often led to severe and rapid battery performance deterioration. The performance fade is mainly due to the volume change-induced pulverization of Si and the inability of the binder/conductive additive to maintain the electrical contact between the pulverized Si particles and the current collector. In addition, the intrinsic low electric conductivity of silicon is another challenge that needs to be addressed.
Although several high-capacity anode active materials have been found (e.g., Si), there has been no corresponding high-capacity cathode material available. Current cathode active materials commonly used in Li-ion batteries have the following serious drawbacks:                (1) The practical capacity achievable with current cathode materials (e.g. lithium iron phosphate and lithium transition metal oxides) has been limited to the range of 150-250 mAh/g and, in most cases, less than 200 mAh/g.        (2) The insertion and extraction of lithium in and out of these commonly used cathodes rely upon extremely slow solid-state diffusion of Li in solid particles having very low diffusion coefficients (typically 10−8 to 10−14 cm2/s), leading to a very low power density (another long-standing problem of today's lithium-ion batteries).        (3) The current cathode materials are electrically and thermally insulating, not capable of effectively and efficiently transporting electrons and heat. The low electrical conductivity means high internal resistance and the necessity to add a large amount of conductive additives, effectively reducing the proportion of electrochemically active material in the cathode that already has a low capacity. The low thermal conductivity also implies a higher tendency to undergo thermal runaway, a major safety issue in lithium battery industry.        
Low-capacity anode or cathode active materials are not the only problem that the lithium-ion battery industry faces. There are serious design and manufacturing issues that the lithium-ion battery industry does not seem to be aware of, or has largely ignored. For instance, despite the high gravimetric capacities at the electrode level (based on the anode or cathode active material weight alone) as frequently claimed in open literature and patent documents, these electrodes unfortunately fail to provide batteries with high capacities at the battery cell or pack level (based on the total battery cell weight or pack weight). This is due to the notion that, in these reports, the actual active material mass loadings of the electrodes are too low. In most cases, the active material mass loadings of the anode (areal density) is significantly lower than 15 mg/cm2 and mostly <8 mg/cm2 (areal density=the amount of active materials per electrode cross-sectional area along the electrode thickness direction). The cathode active material amount is typically 1.5-2.5 times higher than the anode active material. As a result, the weight proportion of the anode active material (e.g. graphite or carbon) in a lithium-ion battery is typically from 12% to 17%, and that of the cathode active material (e.g. LiMn2O4) from 17% to 35% (mostly <30%). The weight fraction of the cathode and anode active materials combined is typically from 30% to 45% of the cell weight.
As a totally distinct class of energy storage device, sodium batteries have been considered as an attractive alternative to lithium batteries since sodium is abundant and the production of sodium is significantly more environmentally benign compared to the production of lithium. In addition, the high cost of lithium is a major issue.
Sodium ion batteries using a hard carbon-based anode (Na-carbon intercalation compound) and a sodium transition metal phosphate as a cathode have been described by several research groups. However, these sodium-based devices exhibit even lower specific energies and rate capabilities than Li-ion batteries. These conventional sodium-ion batteries require sodium ions to diffuse in and out of a sodium intercalation compound at both the anode and the cathode. The required solid-state diffusion processes for sodium ions in a sodium-ion battery are even slower than the Li diffusion processes in a Li-ion battery, leading to excessively low power densities.
Instead of hard carbon or other carbonaceous intercalation compound, sodium metal may be used as the anode active material in a sodium metal cell. However, the use of metallic sodium as the anode active material is normally considered undesirable and dangerous because of dendrite formation, interface aging, and electrolyte incompatibility problems. Most significantly, the same flammable solvents previously used for lithium secondary batteries are also used in most of the sodium metal or sodium-ion batteries.
The low active material mass loading is primarily due to the inability to obtain thicker electrodes (thicker than 100-200 μm) using the conventional slurry coating procedure. This is not a trivial task as one might think, and in reality the electrode thickness is not a design parameter that can be arbitrarily and freely varied for the purpose of optimizing the cell performance. Contrarily, thicker samples tend to become extremely brittle or of poor structural integrity and would also require the use of large amounts of binder resin. The low areal densities and low volume densities (related to thin electrodes and poor packing density) result in a relatively low volumetric capacity and low volumetric energy density of the battery cells.
With the growing demand for more compact and portable energy storage systems, there is keen interest to increase the utilization of the volume of the batteries. Novel electrode materials and designs that enable high volumetric capacities and high mass loadings are essential to achieving improved cell volumetric capacities and energy densities.
Therefore, there is clear and urgent need for lithium and sodium batteries that have high active material mass loading (high areal density), high electrode thickness without significantly decreasing the electron and ion transport rates (e.g. with improved conductivity), high capacity, high power density, and high energy density. These batteries must be produced in an environmentally benign manner.